Research and development on magneto-optical recording media such as magneto-optical disks have been carried out as being a rewritable optical disk, and some of the magneto-optical recording media have been already practically used as external memory designed for computers. Since such a magneto-optical disk adopts a perpendicular magnetization film as a recording medium and recording and reproducing are carried out by using a light, it has a larger capacity compared to a floppy disk or a hard disk adopting an in-plane magnetization film.
In recent years, since recording with higher density is required, a high density magneto-optical recording medium composed of a magnetic layer having a multi-layered structure, which is capable of reproducing a recording bit fairly smaller than a size of a light spot with magnetic super resolution, namely, is capable of reproducing with high resolution, is proposed.
For example, Japanese Unexamined Patent Publication 5-81717/1993 (Tokukaihei 5-81717) discloses a magneto-optical recording medium which is provided with a recording layer for recording information magneto-optically and a readout layer in which in-plane magnetization appears at room temperature and a transition occurs from in-plane magnetization to a perpendicular magnetization when a temperature is risen.
In such a magneto-optical recording medium, reasons that reproducing with high resolution is possible will be explained referring to FIGS. 21 through 23. Here, the case of a magneto-optical disk will be taken.
As shown in FIG. 21, the magneto-optical disk has a basic arrangement that a substrate 201, a transparent dielectric layer 202, a readout layer 203, a recording layer 204, a protective layer 205 and an overcoat layer 206 are laminated in this order.
FIG. 22 shows a magnetic phase diagram of rare-earth transition metal alloy used in the readout layer 203. In the drawing, a horizontal axis indicates a content of rare earth metal (ratio) (RE), and a vertical axis indicates temperature. A composition range where the rare earth metal alloy has perpendicular magnetization (indicated by A in the drawing) is extremely narrow. This is because perpendicular magnetization appears only in the vicinity of a compensating composition (indicated by P in the drawing) where a magnetic moment of the rare earth metal and a magnetic moment of transition metal balance with each other.
The respective magnetic moments of the rare earth metal and the transition metal have different temperature dependencies. Specifically, the magnetic moment of the transition metal becomes greater than that of the rare earth metal at high temperature. For this reason, the content of the rare earth metal is set greater than that in the compensating composition at room temperature so that the alloy does not have perpendicular magnetization but has in-plane magnetization at room temperature. When a light beam is projected, as a temperature of the portion illuminated with the light beam is raised, the magnetic moment of the transition metal becomes relatively greater until it balances with that of the rare earth metal, thereby having perpendicular magnetization.
FIGS. 23(a) through 23(d) show one example of hysteresis characteristics of the readout layer 203. In each drawing, a horizontal axis indicates an external magnetic field (Hex) to be applied perpendicularly to a surface of the readout layer 203, and a vertical axis indicates a polar Kerr rotation angle (.theta.k) in the case where a light is allowed to perpendicularly enter the surface of the readout layer 203. FIG. 23(a) shows hysteresis characteristics of the composition P shown in the magnetic phase diagram in FIG. 22 of the readout layer 203 in a temperature range of room temperature to temperature T.sub.1, and FIGS. 23(b) through 23(d) respectively show hysteresis characteristics in temperature ranges of T.sub.1, to T.sub.2, T.sub.2 to T.sub.3 and T.sub.3 to Curie temperature (Tc).
In the temperature range of T.sub.1 to T.sub.3, the readout layer 203 shows such a hysteresis characteristic that an abruptly rising of polar Kerr rotation angle appears with respect to the external magnetic field. In other temperature ranges, however, the polar Kerr rotation angle is substantially zero.
When the rare earth transition metal having the above-mentioned characteristics is applied to the readout layer, the magneto-optical disk is provided with higher recording density. Namely, a recording bit with a size smaller than a size of a light spot can be reproduced. The reasons for this will be described below.
As shown in FIG. 21, when reproducing, a reproducing light beam 207 is projected as a light spot 209 to the readout layer 203 through a converging lens 208 from a side of the substrate 201. Here, information is recorded on the recording layer 204 in the magnetization direction shown in the drawing, for example. In a portion of the readout layer 203 where the light spot 209 has been projected, its center portion is heated to higher temperature than a peripheral portion. More specifically, since the reproducing light beam 207 is converged to a diffraction limit by the converging lens 208, a light intensity distribution of the light spot 209 shows a Gaussian distribution, and thus a temperature distribution of the portion of the magneto-optical disk which is reproduced also shows like a Gaussian distribution. Here, the light beam 207 is projected to the readout layer 203 such that the temperature of the central portion of the irradiated area in the readout layer 203 is raised above T.sub.1 in FIG. 22 and the temperature of the peripheral portion is not raised above T.sub.1. Since only the portion having a temperature of not less than T.sub.1 is subject to reproduction, a recording bit with a size smaller than a diameter of the light spot 209 can be reproduced, thereby remarkably improving the recording density.
In other words, a transition occurs in the portion having the temperature of not less than T.sub.1 from in-plane magnetization to perpendicular magnetization (from the state shown in FIG. 23(a) to FIG. 23(b) or the state shown in FIG. 23(c)). At this time, a direction of the magnetization of the recording layer 204 is transferred to the readout layer 203 by exchange coupling force between the readout layer 203 and the recording layer 204. Meanwhile, in the peripheral portion other than the central portion where the light spot 209 has been projected, since the temperature is not more than T.sub.1, the in-plane magnetization (FIG. 23(a)) is maintained. As a result, with respect to the light beam 207 irradiated in a direction perpendicular to the film surface, the polar Kerr rotation effect is not shown.
As described, when a transition occurs in the portion where the temperature is risen from in-plane magnetization to perpendicular magnetization, the pole Kerr rotation effect is shown only in the central portion where the light sport 209 has been projected, and only information recorded on the recording layer 204 corresponding to the above portion is reproduced based upon a reflected light from the portion.
Thereafter, when the light spot 209 shifts (the magneto-optical disk rotates) so that the next recording bit is reproduced, the temperature drops below T.sub.1 in the portion which has been previously subject to reproduction, and a transition occurs from perpendicular magnetization to in-plane magnetization. Accordingly, the polar Kerr rotation effect is no longer shown in the portion where the temperature drops. Therefore, information in the portion where the temperature drops is no longer reproduced and thus interference by signals from the adjoining bits, which causes noise, is eliminated.
As mentioned above, when the above magneto-optical disk is used, a recording bit with a size smaller than the diameter of the light spot can be securely reproduced without being affected by the adjoining recording bits, so the recording density can be remarkably improved.
As to the embodiment of the high density magneto-optical recording disk, the inventors of the invention disclose the properties and effects in the Proceedings of Magneto-Optical Recording International Symposium 1992, J. Magn. Soc. Jpn., Vol. 17, Supplement No. S1 (1993), pp. 201-204, "Super Resolution Readout of a Magneto-Optical Disk with an In-plane Magnetization Layer".
In the publication, in the magneto-optical recording medium having the arrangement shown in FIG. 21, GdFeCo is used as the readout layer 203 and DyFeCo as the recording layer 204. In the GdFeCo, its composition range that perpendicular magnetization (shown by A in FIG. 22) is extremely narrow and a transition suddenly occurs from in-plane magnetization to perpendicular magnetization with respect to the temperature. Therefore, the GdFeCo is a suitable material for the high density recording medium. FIG. 24 shows a temperature dependency of the polar Kerr rotation angle measured from the readout layer of the recording medium. A threshold temperature at which a transition occurs from in-plane magnetization to perpendicular magnetization is around 100.degree. C. At temperature not more than 100.degree. C., since in-plane magnetization is shown, the polar Kerr rotation angle is extremely small. Meanwhile, since a transition suddenly occurs from in-plane magnetization to perpendicular magnetization in the vicinity of 100.degree. C., the polar Kerr rotation angle suddenly increases.
The threshold temperature is a very important factor which determines the reproducing laser power at the time of reproducing signals by the laser beam. FIG. 25 shows a relationship between the reproducing laser power and an amplitude of the reproducing signal in the magneto-optical recording medium. The amplitude of the signal suddenly increases with an increase in the reproducing laser power, and is maximized at about 2 mW to 2.3 mw.
From the above-mentioned principle of reproduction, a signal can be obtained only after the area having a temperature above 100.degree. C. appears in the reproducing light spot. Moreover, in the case where the threshold temperature is 100.degree. C., a suitable reproducing power is 2 mW to 2.3 mW.
In the case where the threshold temperature is higher than 100.degree. C., more reproducing power is required. If the threshold temperature is set too high, unwanted recording may occur by the reproducing power, and information recorded on the recording layer may be destroyed. On the contrary, when the threshold temperature is set lower than 100.degree. C., a lesser reproducing power is required. However, if the threshold temperature is set too low, to about 40.degree. C., for example, the temperature of the whole spot becomes not less than 40.degree. C. in the case where a circumferential temperature at the time of reproduction is 40.degree. C., thereby making it impossible to reproduce information with high resolution.
As mentioned above, in the high density magneto-optical recording medium, it is very important to control the threshold temperature at which a transition occurs from in-plane magnetization to perpendicular magnetization.
In addition, at the time of reproduction, in an area having a temperature above the threshold temperature, the magnetization direction of the recording layer (in a direction perpendicular to the film surface, upward or downward) should be securely transferred to the readout layer. In other words, the portion where the magnetization direction of the readout layer is perpendicular should follow the magnetization direction of the recording layer (upward or downward).
The above conditions can be achieved by controlling each composition of the layers considering the magnetic interaction between the two magnetic layers which are the readout layer and the recording layer.
In addition, in order to stabilize the recorded information, the recording layer is required that coercive force at room temperature is large and required to have a Curie temperature that an extremely large laser power is not required for recording.
In addition, it is not an object to make it possible to reproduce information with high resolution by magnetic super-resolution described in the above conventional embodiment, but the magneto-optical recording medium, which is arranged such that the magnetic layer of the recording film has a multi-layer structure, has been proposed. For example, Japanese Examined Patent Publication No. 2-35371/1990 (Tokukohei 2-35371) discloses a magneto-optical recording medium which is arranged such that two magnetic layers are laminated each other as a magneto-optical recording medium in which lowering of the recording power does not deteriorate quality of a reproducing signal (S/N ratio) and in which the recorded information is stable with respect to the external magnetic field. The magneto-optical recording medium is composed of (1) the magnetic layer having a high Curie temperature of not less than 200.degree. C. and small coercive force (the magnetic layer corresponding to the readout layer) and (2) the magnetic layer having a low Curie temperature of not more than 200.degree. C. to not less than 50.degree. C. and large coercive force (the magnetic layer corresponding to the recording layer). The magnetic layer (1) is made of amorphous alloy containing Gd-Fe or Gd-Co, and the magnetic layer (2) is made of amorphous alloy containing Tb-Fe or Dy-Fe. The magnetic layer having large coercive force and the magnetic layer having small coercive force are exchange-coupled.
As one of the effects of this arrangement, since the magnetic layer having small coercive force and high Curie temperature which is exchange-coupled with the magnetic layer having large coercive force exists, information is read from the magnetic layer having small coercive force, so a desirable S/N ratio can be obtained at the time of reading out.
In addition, a magneto-optical recording medium where a composition of the magnetic layer is inclined has been proposed. Such a magneto-optical recording medium is disclosed in for example, Japanese Unexamined Patent Publications Nos. 54-121719/1979 (Tokukaisho 54-121719), 63-282945/1988 (Tokukaisho 63-282945) and 5-325283/1993 (Tokukaihei 5-325283). In Japanese Unexamined Patent Publication No. 54-121719/1979, GdCo is used in order to stabilize a, writing recording bit thermally and magnetically, and the composition of the magnetic layer is changed in a direction of a film thickness, as an example.
In Publication No. 63-282945/1988, TbFeCo is used as a material of the magnetic layer composed of a single layer, and the composition contains a lot of Tb and a little Co on the light incidence side. As a result, the Kerr rotation angle on the light incidence side becomes large.
In Publication No. 5-325283/1993, as a technique which uses a composition tilt film as a magnetic multi-layer film, a technique which improves sensitivity of a magnetic field by using a two-layer film made of TbFeCo while stability of recording is being maintained.
In addition, in the case where the recording density of the magneto-optical recording medium is further improved, it is desired that a laser beam to be used has a short wavelength, but in an alloy film composed of heavy rare earth metal and transition metal, such as GdFeCo, as the wavelength becomes shorter, the polar Kerr rotation angle (.theta.K) becomes smaller, and thus the quality of the reproducing signal (C/N) is deteriorated.
Therefore, the following method is generally used. Namely, (1) as described in Journal of Japanese Applied Magnetics Vol. 12, No. 2, 1988, pp. 207-210, in order not to decrease the polar Kerr rotation angle or in order to increase it even when the wavelength is short, the alloy film composed of light rare earth metal and transition metal is used. The journal describes that when Nd-(Fe, Co) alloy is used, the polar Kerr rotation angle is not decreased or it is increased even when the wavelength is short.
(2) In addition, as described in Abstract of one lecture of the 17th lecture meeting held by Applied Magnetics association 10aC-6, the alloy composed of heavy rare earth metal, light rare earth metal and transition metal is used for the reason same as of (1). The journal describes that when the Nd is added to the the TbFe, decrease in the polar Kerr rotation angle becomes small in a range of 400 nm to 600 nm.
As mentioned above, it is suggested that when the density of recording is made higher by using a laser beam having a short wavelength, light rare earth metal is added to the readout layer in order to prevent lowering of the quality of the reproducing signal due to the decrease in the polar Kerr rotation angle, but the magneto-optical recording medium where only the light rare earth metal is simply added to the readout layer has the following problems.
Namely, as disclosed in the publication No. 2-35371/1990, for example, in the case of the magneto-optical recording medium which records and reproduces information using the magnetic interaction between two layers, when the light rare earth metal is added to the readout layer, the magnetic layer having small coercive force (readout layer) where an entire magnetic moment has been changed is laminated on the magnetic layer having large coercive force (recording layer), so exchange-coupling force between the readout layer and the recording layer changes. Therefore, when the light rare earth metal is added to the readout layer, the exchange-coupling force changes, thereby arising a problem that the recording and reproducing operations are not carried out properly.
In addition, for example, as disclosed in Japanese Unexamined Patent Publication No. 5-81717/1993 (Tokukaihei 5-81717), in the case of a magneto-optical recording medium which records and reproduces information with high density by utilizing a transition from in-plane magnetization to perpendicular magnetization due to rise in temperature of a readout layer, a magnetic moment of light rare earth metal becomes parallel with a magnetic moment of transition metal. For this reason, when light rare earth metal is added to the readout layer, the magnetic moment of the heavy rare earth metal balances with the magnetic moment of the light rare earth metal--the transition metal, so there is danger of shifting of the threshold temperature to a low temperature. If the threshold temperature becomes too low, there arises a problem that desirable reproducing with high resolution cannot be realized.
In addition, in the magneto-optical recording medium, when the magnetic moment of the light rare earth metal--the transition metal has been already superior to the magnetic moment of the heavy rare earth metal at room temperature, basic properties do not appear such that in-plane magnetization appears at room temperature, whereas a transition occurs from in-plane magnetization to perpendicular magnetization when the temperature rises, thereby deteriorating the basic operation.